Biologists generally accept the first step, or rather organism, of angiosperm evolution as being cyanobacteria. Cyanobacteria
are aquatic, photosynthetic bacteria that are sometimes called by the more deceptive name of blue-green algae. As they are
bacteria, they are also prokaryotic and usually unicellular organisms. Unlike most bacteria, however, cyanobacteria often
grow in large colonies that can actually be large enough to be visible to the unaided human eye.

The process of endosymbiosis supports the concept of cyanobacteria. The theory behind endosymbiosis is that cyanobacteria
evolved into chloroplasts after other eukaryotic cells absorbed them as endosymbionts (the technical name for any organism
that lives with the body or cells of another organism). Now that the cyanobacteria are no longer individual cells and are
instead chloroplasts within another organism, it is time to introduce the next organism that is part of this proposed sequence
for angiosperm evolution: chlorophyta.

The origin of flowering plants has been one of the most famous and contentious
issues in evolutionary biology. Based on data from living plants, the “Mostly Male” theory was developed. This
theory serves to explain that developmental control of flower organization derives more from systems active in the male reproductive
structures of the gymnosperm ancestor, rather than from the female, with ovules being ectopic in the original flower:
The mostly-male theory is based on observations of the duplication, in seed plants, of a nuclear gene associated with
reproductive morphogenesis and the subsequent loss, in flowering plants, of one of the two copies of this gene. Simplifying
the original theory greatly, the argument can be outlined as follows: ovules, usually sterile ones, may be produced ectopically,
i.e. in positions where they are not expected to occur. Ectopic ovules on microsporangiate strobili could have replaced the
function of female strobili, if these were lost as a result of the ancestor of flowering plants losing the duplicate gene
referred to above. As suggested at the right, an angiosperm pistil could have resulted from an ovule-bearing but otherwise
sterile microsporophyll (the placenta) becoming enclosed by another sterile microsporophyll (the ovary wall). Perianth members
might have developed by sterilization of the most basal microsporophylls. (Dickinson)
The idea behind the evolution of gnetophyta to angiosperms is that the two distinct leaves that are present within gnetophyta
are comparable to the two cotyledons that are present within the dicotylodonea group of angiosperms. This explanation of
the evolution of angiosperms does not yet totally answer the question of the way in which angiosperms have evolved into the
way that they appear today. To take this into account, the theory that this explanation is based off of states that through
various mutations that dicotylodonea group of angiosperms could eventually diverge to form the lilliopsida group of angiosperms
which has only a single cotyledon. The converse of this theory is also held to be plausible; that lilliopsida evolved first
and then mutated to become dicotylodonea.